Question: Which of these numbers is composite? ${19,\ 31,\ 39,\ 47,\ 79}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 19, 31, 47, and 79 each have only two factors. The factors of 39 are 1, 3, 13, and 39. Thus, 39 is the composite number.